ISABELLA M. GIOIA

I review some of the current efforts to create unbiased samples of galaxy clusters. Readers are referred elsewhere for general wide area sky surveys and redshift surveys, and for Sunyaev-Zeldovich, radio, infrared and submm surveys, some of which were not designed to search primarily for clusters. My focus will be on optical and X-ray samples and on high redshift clusters.

L.T. BARONE et al.

After the completion of the catalogue built with ESO key program (REFLEX), we are beginning to explore the multiband optical characteristics of a subsample of those clusters selected in a statistically independent way. We have already observed in B, V, R, 9 of the about 50 clusters of the subsample, with $0.15 \le z \le 0.18$. More time has been allocated by ESO for our project in March 2001. We will therefore be able to compare X-ray and optical morphologies and luminosities and, thanks to the large field of view of the WFI instrument, assess the luminosity function and the color segregation in the cluster on a robust statistical basis. In this poster we present the project, the technical problems we are facing in the reduction phase, and show the first preliminary results, which seem very encouraging.

WAYNE H. BAUMGARTNER et al.

The ASCA Cluster Project (ACP) is a compilation of data for over 200 galaxy clusters and groups observed with the ASCA X-ray satellite. We homogeneously reduce and fit data from the GIS and SIS instruments on ASCA to obtain isothermal temperatures, redshifts, luminosities, and abundances for the clusters. We show that the x-ray derived cluster redshift is a good match to the optically derived redshift for clusters with temperatures above 3.0~keV, and that the luminosity--temperature relation shows no downturn at lower temperatures. Trends in cluster metal abundance with temperature are discussed.

C. MENDES DE OLIVEIRA et al.

We are undertaking a large, wide-field survey of nearby groups and poor clusters. The main goals are identifying and characterizing the properties of low-surface-brightness dwarf galaxies and determining the galaxy luminosity function for M$_R > -17$. Large areas (typically 0.3--0.5 degree$^2$ per system but up to 7 degree$^2$) of the groups Telescopium, Leo I, Dorado, N5575, HCG 42, HCG 44, HCG 68 and the poor clusters IC 4765 and Hydra have so far been surveyed in V and I. We present the preliminary results for the photometric study of the groups HCG 42, Telescopium (or NGC6868) and IC4765. Hundreds of new low surface brightness galaxies are catalogued. Their spatial distributions, colors, types and sizes will be studied as a function of the richness of their environments.

K. L. THOMPSON, J. A. WILLICK and B. F. MATHIESEN

We present a progress report for . Our aim is a complete catalog of optically selected clusters in the range $0.3 <= z 1.0$ with well-understood selection effects to derive $n(M,z)$ for cosmological and evolutionary studies. cluster candidates are found on existing deep images, and we use the Hobby-Eberly Telescope to confirm redshifts and measure velocity dispersions. The catalog will be made available to the community for follow-up observations of the X-ray properties, the S-Z effect, and weak and strong lensing. Multiwavelength observations are necessary for the derivation of accurate mass profiles which are needed to model the cluster mass function accurately.

P.T.P. VIANA et al.

We have modelled the expected properties of an XMM-Newton serendipitous cluster survey for three different cosmological models, using the Extended Press--Schechter framework. We estimate that, over the ten year design lifetime of XM}, the EPIC camera will image a total of around 800 square degrees in fields suitable for the serendipitous detection of clusters of galaxies. For the presently-favoured low-density model with a cosmological constant we predict that this survey would yield a catalogue of more than 8000 sources, ranging from poor to very rich clusters, with around 750 detections above z=1. A low-density open Universe yields similar numbers, though with a different redshift distribution, while a critical-density Universe gives considerably fewer clusters. The catalogue resulting from an XMM serendipitous cluster survey would facilitate a variety of follow-up projects, including the quantification of evolution in the X-ray luminosity-temperature relation, the study of high-redshift galaxies via gravitational lensing and the analysis of foreground contamination in cosmic microwave background maps. Most importantly, the catalogue will allow stringent constraints to be placed on the values of the total amount of matter in the Universe and of a possible cosmological constant.